ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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Testing Methods for Solid Friction

K.G. Budinski, Eastman Kodak Company; P.J. Blau, Oak Ridge National Laboratory

Introduction

IN MACHINERY WITH MOVING PARTS, friction control can make the difference between acceptable

operation and catastrophic failure. Friction in moving parts also reduces the energy-efficiency of machines,

making them more costly to operate than if friction were reduced. Therefore, the measurement of friction force

and the calculation of the coefficient of friction is warranted for many kinds of machines. In machine design,

friction characteristics must be known for friction drives, for sizing motors, for determining work forces, for

braking, for rotary seals, and for designing any part of the machine in which changes in forces between sliding

members could affect operation. There is a close parallel between the history of friction science and the

development of transportation systems, because friction problem solving has enabled the development of all

kinds of engines, motor bearings, brakes, transmissions, seals, tires, and other vehicle components (Ref 1). To

emphasize the importance of friction in machine design, it has been estimated that one-third to one-half the

energy production of the world is wasted through friction (Ref 2).

Friction is not a measure of wear or the tendency to wear; therefore, it does not determine service life.

Frictional heating sometimes causes a part to fail, but this usually occurs when circumstances cause abnormal

friction effects. Articles describing a variety of measurement methods for different types of wear can be found

in other articles in this Section of the Handbook and in Friction, Lubrication, and Wear Technology, Vol 18,

ASM Handbook.

This article describes techniques for measuring friction. Emphasis is on solid/solid sliding friction, but in

several instances, lubricated testing methods are described. Many different methods have been developed to

measure friction because frictional behavior is dependent not only on the properties of the materials involved,

but also on the manner is which they come together and interact (Ref 4). The objective of this article, therefore,

is to provide enough information so that a person who has never conducted a friction test can do so. Depending

on the method of measurement and the way in which the data are to be used, friction data can be obtained in

various forms. The last section of this article therefore, describes how to report friction data and how these data

can be entered into a database.

References cited in this section

1. D. Dowson, History of Tribology, Longman, 1979

2. A.Z. Szeri, Tribology: Friction, Lubrication, and Wear, Hemisphere Publishing Corp., 1980, p 2

4. P.J. Blau, Friction Science and Technology, Marcel Dekker, Inc., 1996

Testing Methods for Solid Friction

K.G. Budinski, Eastman Kodak Company; P.J. Blau, Oak Ridge National Laboratory

Definition of the Friction Coefficient

The static friction force, F

, is that force which is just sufficient to resist the onset of relative motion between

two bodies in contact under an applied normal force, N. The kinetic friction force, F

, is that force which must

be overcome to allow the continuance of motion between two bodies. F

and F

act along or tangent to contact

surface or interface in a direction so as to directly oppose relative motion. Thus, the static friction coefficient,

, and the kinetic friction coefficient, μ

, can be defined as a dimensionless ratio of forces as follows:

(Eq 1)

and

(Eq 2)

Elsewhere in this article, if there is no subscript for μ and it is not otherwise stated, it should be assumed that μ

is implied.

In Eq 1 and 2, both N and F are expressed in units of force. For example, a 1 kg mass resting on a plane exerts a

downward force of 9.81 newtons. Strictly speaking, the applied “load” is only equivalent to the normal force if

it is applied downward. Thus, it is appropriate to say a load of 9.8 newtons was applied only if the weight acts

downward, as in deadweight loading. If, as in a horizontally loaded component, the N acts in a direction other

than parallel to the gravitational force, the expression “normal force,” not “load,” must be used. In practice, it is

always better to use the general term “normal force” to avoid having to make this distinction.

Testing Methods for Solid Friction

K.G. Budinski, Eastman Kodak Company; P.J. Blau, Oak Ridge National Laboratory

Methods for Measuring Friction

Surface Preparation. Before one conducts any kind of friction test, surfaces of the test pieces should be properly

prepared, and their method of preparation should be documented. Since frictional behavior reflects the

condition of the contact surfaces and the interface conditions between them, it can be affected by the initial

cleanliness of surfaces as well as the method used to produce those surfaces (i.e., machining conditions,

polishing methods, etc.). The degree to which the cleanliness and other initial preparation factors will affect the

results depends to some extent on the applied test conditions. For example, lightly loaded contacts tend to be

more sensitive to surface cleanliness than heavily loaded surfaces in which the onset of sliding quickly wipes

films away and can even alter the surface roughness and the subsurface microstructure.

In preparing surfaces for application-oriented testing, including lubricated testing, the conditions of the

tribosystem should be simulated as closely as possible. For example, if brake rotor surfaces become rusty

during long idle periods in a parking lot, then rusty surfaces should be used for brake rotor friction testing to

simulate this condition. Absorbed films from the atmosphere can be another source of contamination. In tests of

ultrahigh-molecular weight polyethylene used for conveyor tracks, for example, it was found that if the material

sat unused for 48 h or more, as over a weekend, the starting friction was unacceptably high due to the formation

of a film on the material. Thus, the laboratory test conditions should mimic such practical operating

considerations.

The effects of specimen surface preparation and handling are not further discussed here, but the reader can

consult Ref 2 and 3 for additional details. The next sections describe friction testing geometries and the major

considerations implicit in their use.

Inclined-Plane Method. Most commonly used friction coefficient measuring methods require the measurement

of both the friction force and the normal force, but not all friction coefficient measuring methods do. For

example, the inclined-plane static friction coefficient measurement method depicted in Fig. 1 and dating back

over 400 years to Leonardo da Vinci's notebooks, requires only a block of material resting on a flat surface,

which can be tilted gradually. The weight of the block need not be known, only the angle of tilt. In this case, μ

is obtained from the following expression:

= tan θ

(Eq 3)

The specimen is placed on the plane in a horizontal position, and the plane is gradually raised. At some angle,

θ, movement of the specimen begins, and the static friction coefficient can be determined. This simple and

straightforward test lends itself to standardization. The ease of repetition allows many measurements to be

made to obtain an average and standard deviation for μ

. If the static friction coefficient is relatively low (less

than 0.09, θ = ~5°), additional precautions in leveling and measuring the tilt angle are required. In fact,

alternative means for friction measurement should be considered if such low values are to be accurately

measured.

Fig. 1 Inclined plane measurement of the static coefficient of friction (μ

)

Inclined-plane tests form the basis for ASTM D 3248 (for measuring cardboard friction) and D 3334 (for

measuring the friction of woven fabrics), but they can be used for a wide variety of other solid materials. This

method does not require the weight of slider block to be known, but research on metals has shown that the

results are not independent of the normal force, especially at light loads (Ref 5). The problem with this

technique is that it does not simulate many real-life tribosystems. Machine designers do not design systems that

involve only inclined planes, and friction is not dependent on just the materials making up the couple.

Therefore, more complex machines that simulate the frictional contact of the specific applications can be

required. The same argument holds true for simple pulley and weight methods described in the next section.

Friction Test Methods Using Weights and Pulleys. The friction coefficient can be calculated for simple

geometries using arrangements of weights and pulleys. Some typical arrangements for measuring static friction

coefficients are shown in Fig. 2. Mass is added until the block or cylinder of known weight just begins to move.

Equation 1 can then be used.

Fig. 2 Schematics of friction-measuring devices based on early work of Leonardo da Vinci

Like the inclined plane method, the problem with block and pulley methods is that the precision and

repeatability of the measurement tends to be not very high. One has to account for such things as the stretching

of the connecting wires or cords, the friction in the pulleys, pulling any knots tight before motion begins, and

more. It is thus, desirable when possible to use force sensors. With force sensors and smooth motor drives, sled-

type, block-on-plane methods can be used to measure not only the static but the kinetic friction coefficient as

well.

Pulling a block on a flat surface with a load sensor yields the friction force, F in the basic friction equation, F =

μN, where N is the normal force. This is probably the most common method of measuring friction force.

Although there are countless variations of this test, most devices use a force sensor that is essentially a

calibrated spring. This spring has a spring constant that determines the sensitivity of the system; there is

probably some damping associated with the system. A free-body diagram of a sled friction tester is shown in

Fig. 3.

Fig. 3 Free-body diagram of a sled friction tester. N, normal force

Certain system parameters can affect the results of a friction study. All that is measured is the friction force, F,

by means of an electrical signal or a mechanical device. Both types of sensors have elastic and damping

characteristics that can affect the measurement. The major concern is static friction. If the pulling system shown

in Fig. 3 were replaced by a steel rod, which would have much smaller time-dependent strain behavior, the

system model could be simplified as shown in Fig. 4. The measured force would still involve the elastic

properties of the steel rod (spring a), but any time-dependent component of the friction force would come from

the system under study, that is, the elasticity, a, and viscous behavior, b, of the surface contacts.

Fig. 4 Free-body diagram of a sled friction tester with a nonelastic driver.

Friction Tests of Shafts and Capstans. The friction of shafts turning in journal bearings and of flexible materials

sliding over curved surfaces represents two important classes of practical frictional problems. In each case, the

plane of interaction is not flat, and the resistance to rotation is of interest. The directions of the friction force

represent a family of vectors of tangency to the curved surface acting along the contact zone.

Figure 5 shows a diagram of a journal bearing. The Petrov equation, developed in the late 19th century, can be

used to calculate the friction coefficients as follows:

(Eq 4)

where r is shaft radius, l is bushing length, c is lubricant film thickness, η is lubricant viscosity, ω is shaft

velocity, and N is normal force.

Fig. 5 Petrov equation for the coefficient of friction of a lubricated tribosystem. See text for details.

The friction of a journal bearing or rotating bushing can be measured using an arrangement such as that shown

in Fig. 6. The use of a calibrated spring of some kind (as in Fig. 4) is probably the most common method of

measuring friction force. The spring has a spring constant that determines the sensitivity of the system, but as

shown in Fig. 3 and 4, there is probably some damping associated with this kind of system as well.

Fig. 6 Use of load cells for measurement of friction force

In another case of non-flat sliding contact, a flexible web is wrapped around a capstan. Figure 7 shows this

geometry. The tension T

, which can be measured by a force sensor, is given by the following:

= We

μb

(Eq 5)

where W is the weight (or another tension force), μ is the friction coefficient (either static or kinetic depending

on the use for the test), b is the wrap angle around the capstan or shaft (in radians), and e is the base of natural

logarithms (2.7183). Solving for μ as a function of the measured tension, applied tension or weight, and the

wrap angle in radians is possible, as follows:

(Eq 6)

The capstan test has become a standard ASTM test method ASTM G 143. When using multiple wraps, as in

ASTM D 3412, the value of b is adjusted accordingly to equal the number of wraps, or wraps plus fractions of

complete wraps, times 2 radians.

Fig. 7 Schematics of friction-measuring concept for non-flat sliding contact

Other Types of Friction Tests, Including Standards. As mentioned, earlier, there are many types of friction tests

because there are many reasons for obtaining friction data. In some cases, one might wish to simulate a certain

tribosystem. In other cases, one might wish to screen new materials, lubricants, or coatings in a more generic

sense and without a specific application in mind. Sometimes, the fine-scale frictional interaction of surfaces is

studied purely for scientific purposes.

Sometimes, only a limited quantity of a new or experimental material or lubricant is available, and it is

necessary to use small, easy-to-fabricate test specimens. A variety of commercial friction testing machines is

available for friction testing using simple test geometries, like block-on-ring and pin-on-disk.

Table 1 shows ASTM friction test methods. Standards documents generally describe the characteristics of a

method, as well as its applicability, precision, and accuracy.

Table 1 Selected solid friction tests per ASTM standards

Standard (committee), title Material couple

(measured parameters)

Test configuration

B 460 (B-9 on metal powders), Dynamic

Coefficient on Friction and Wear of Sintered

Metal Friction Materials under Dry

conditions

Friction materials versus

metal (μ

versus

temperature)

B 461 (B-9 on metal powders), Frictional

Characteristics of Sintered Metal Friction

Materials Run in Lubricants

Friction materials versus

metal (μ

versus number

of engagements) (μ

versus velocity)

B 526 (B-9 metal powders), Coefficient of

Friction and Wear of Sintered Metal

Friction materials versus

gray case iron (μ

and μ

)

C 808 (C-5 on carbon/graphite), Reporting

Friction and Wear Test Results of

Carbon versus other

materials (μ

and μ

)

Any apparatus

Manufactured Carbon and Graphite Bearing

and Seal Materials

D 1894 (D-20 on plastics), Static and

Kinetic Coefficients of Friction of Plastic

Films and Sheeting

Plastic film versus stiff or

other solids (μ

and μ

)

D 2047 (D-21 on polishes), Static

Coefficient of Friction of Polish Coated

Floor Surfaces as Measured by the James

Machine

Flooring Materials versus

shoe heels and soles (μ

and μ

)

D 2394 (D-7 on wood), Simulated Service

Testing of Wood and Wood-Base Finish

Flooring

Wood and wood-base

flooring versus sole

leather (μ

and μ

)

D 2714 (D-2 on lubricants), Calibration and

Operation of Alpha Model LFW-1 Friction

and Wear Testing Machine

Steel ring versus steel

block (lubricated with

standard oil) (μ

)

D 3028 (D-20 on plastics), Kinetic

Coefficient of Friction of Plastic Solids and

Sheeting

Plastic sheets or solids

versus other solids (μ

and μ

)

D 3108 (D-13 on textiles), Coefficient of

Friction Yarn to Solid Material

Textile yarns versus

solids (μ

)

D 3247 (D-6 on paper), Coefficient of Static

Friction of Corrugated and Solid Fiberboard

Self-mated cardboard (μ

)

D 3248 (D-6 on paper), Coefficient of Static

Friction of Corrugated and Solid Fiberboard

(inclined plant method)

Self-mated cardboard (μ

)

D 3334 (D-13 on textiles), Testing of

Fabrics Woven from Polyolefin

Monofilaments

Self-mated woven fabric

(μ

)